Automotive alternators have been traditionally limited to use as pure electric generators where a voltage regulator modulates a small field current to produce a constant DC output voltage. Today, in addition to regular generating function these machines can also be used as motors to provide torque for tasks such as driving assist or engine starting. Voltage regulation as well as other motor functions are now a function of an inverter system known as an ECU (Electronic Control Unit). To realize these additional functions many alternators now utilize field oriented control (FOC) to regulate the torque production of an alternator to produce the constant output voltage in generating mode, or a torque demand when in motor mode. The FOC transforms the stator sinusoidal current of a three phase motor as two orthogonal components which can be identified as a vector. The measured phase current is converted to vectors in the coordinate system in the DQ frame. The d-axis component of the stator current vector is used to control the rotor flux linkage and the q-axis component is used to control the motor torque. Decoupled torque and flux currents can be derived for control algorithms development. Controllers such as proportional and integral controller (PI controller) are then used to regulate the phase current with a loop feedback mechanism.
However, some poly-phase electric machines tend to develop a non-sinusoidal back electromotive (BEMF) voltage, especially lower cost claw pole type alternators. These alternators are being adapted for use in automotive start-stop systems, mild/micro hybrid applications, as well as generating. The BEMF voltages are non-sinusoidal and thus contain harmonics of a higher order than the motor fundamental frequency. Due to the nature of the FOC, these harmonics appear in the DQ plane as an AC component frequency shifted by N−1, where N indicates the harmonic order. These harmonics create controllability problems for the control algorithms as they can easily exceed the loop bandwidth. Simple PI controller tuning cannot give adequate performance over the full operating rpm range of the machine due to bandwidth limits inherent with PI controls.
Accordingly, it would be advantageous to have a system which could predict and eliminate the effect in the DQ plane of the non-sinusoidal BEMF voltages.